A general description of wind power generation is to be found for example in the article Blaabjerg and Ned Mohan: Wind power, Wiley Encyclopedia of Electrical and Electronics Engineering, John Wiley & Sons, 1999, volume 23, pages 613–618, which article is hereby incorporated by reference.
A wind power plant usually comprises a plurality of windmills, each comprising a wind turbine mechanically coupled to an electric generator for conversion of the wind power to electric power. The wind turbines are, in dependence on the local wind conditions, distributed over a given area, typically in a number of parallel strings perpendicular to the prevailing wind direction, or where no such wind direction is to be found, in a grid layout.
A power collection system within the wind power plant is formed by coupling the generators along a string to a radial cable running along the string and connecting the radial cables to each other at a so called point of common connection (PCC).
The power generated by the wind power plant is supplied to a load network in the form of an electric power grid, for example a utility grid, having a rated frequency (usually 50 or 60 Hz) and a rated voltage that may typically be at the 132 kV level. Typically, the rated voltage at common connection is 22 kV and the point of common connection is then coupled to the power grid via a high voltage step-up power transformer.
Windmills may be divided into two categorises, i.e. fixed-speed and variable-speed mills, referring to whether the turbine and the rotor of the electric generator will operate at an at least substantially fixed rotational speed, determined by the frequency of the power grid, or operate with a variable rotational speed adapted to the actual wind conditions and the characteristics of the wind turbine.
Fixed-speed windmills may be equipped with some kind of synchronous generators, such as reluctance machines or conventional synchronous machines, but are, due to mechanical design considerations, more often equipped with induction generators.
Induction generators are of an uncomplicated design requiring only a minimum of control equipment, which also makes them attractive from an economical point of view. As they are usually designed with a low number of poles, typically 4 or 6, a mechanical gearbox is required to adapt the low rotational speed of the wind turbine to the speed of the generator.
The control equipment usually comprises only some starting equipment to limit the inrush current when the generator is connected to the power collection system.
However, induction generators cannot inherently generate reactive power, and the reactive power needed for their operation is thus provided by phase capacitors coupled to the stator windings of the generator.
The reactive power consumption of this type of generators is not controllable but dependent on the active power and on the voltage of the generator. Consequently, the exchange of reactive power with the grid to which the generator is coupled will vary substantially in dependence on the load of the generator, and the voltage of the network will exhibit corresponding voltage variations. These voltage variations are particularly considerable when the network is weak, i.e. has a low short circuit capacity.
The operator of the electric power grid usually has a requirement on the maximum level of the voltage supplied from the wind power plant. Usually all the generated electric power is supplied to the grid. In particular with increasing unit sizes of the windmills and with an increasing distance between the wind power plant and the grid, the voltage control at the point of common connection has been identified as a problem. The voltage rise, typically occurring at times of low grid load and high output power from the windmills, is dependent on the short circuit power at the point of common connection and in particular where the wind power plant is equipped with fixed-speed windmills, a situation may arise where it will be necessary to switch off a windmill in order to keep the voltage level within prescribed limits. This of course means an undesirable loss of energy.
Thus, in order to obtain an acceptable voltage control in the network, in particular when the network is weak, a controllable reactive power compensation means is required.
The mechanical torque of the wind turbine is subject to fluctuations, in particular to periodic fluctuations due to the design of the wind turbine, typically at a frequency in the order of 1–2 Hz, occasionally even below 1 Hz. A predominant source of such fluctuations is the so-called vortex interaction. However, for example imperfections in the gearbox may be the cause of fluctuations even in higher frequency ranges, typically in the order of up to 8 Hz.
Although the induction generators of a fixed-speed windmill have some inherent damping, such torque fluctuations will, due the consequential fluctuations in the rotational speed of the induction generator, cause fluctuations in the outputted active power of the generator, and, due to the inherent characteristic of such a generator, also in the reactive power exchange with the power collection system, thereby affecting the voltage quality of the electric power grid.
To obtain a control of reactive power flow that is fast enough to reduce voltage variations in the above mentioned frequency ranges, the reactive power compensation means preferably shall to be of the kind comprising one or more capacitor banks and a controllable inductor coupled in parallel with the capacitor banks.
In a known type of such compensator means, the inductor is connected in series with gate-controlled thyristors coupled in anti-parallel, whereby the susceptance of the inductor is controlled by the firing angle of the thyristors, so-called Thyristor Controlled Reactors (TCR).
However, such compensators generate, due to their operational principle, harmonics, which inter alia requires some kind of harmonic filtering to avoid that the harmonic currents are injected into the connected grid.
As mentioned above, in a wind park, a plurality of windmills are coupled to a so-called point of common connection. The voltage at this point of common connection is usually in the range of 10–30 kV. In cases where the power has to be transmitted over longer distances along a transmission link, the transmission link is preferably arranged to comprise a high voltage step-up transformer for increase of the transmission voltage to the range of 100–500 kV.
This limits the options for connection of a TCR, which, because of the thyristors comprised therein, is usually not connected directly, i.e. without a coupling transformer, to voltages higher than 36 kV.